This invention relates to a system for separating solid particles from a flue gas at high temperatures and pressures and, more particularly, to a separator in which both a centrifugal separation and an electrogasdynamic separation is achieved.
A major constraint on the firing of pulverized coal in electric utility boilers is the collection of large quantities of fly ash. Traditionally, electrostatic precipitators or various types of cyclones have been used to effect the separation of the fly ash from the flue gases.
Cyclone separators are versatile cleanup devices, being applicable to large and small gas flow rates, temperatures up to 2000.degree. F., and pressures exceeding 5 atmospheres. In addition, cyclone separators are relatively insensitive to particle chemistry. However, cyclone separator efficiency decreases markedly for particle sizes below 10 .mu.m.
Electrostatic precipitators can maintain a high efficiency (greater than 95 percent) in a range from less than 1 to 10 .mu.m. However, they are limited to temperatures below 800.degree. F. and pressures in the range of 1 atmosphere, and they are sensitive to particle chemistry. Some of the precipitators have been able to achieve collection efficiencies as high as 99 percent but only at low temperatures (450.degree.-600.degree. F.). Furthermore, since some new regulations require an efficiency in excess of 99.5 percent, and even higher efficiencies are required if the cleaned flue gases are used to drive turbines, these designs have become less than satisfactory, especially since the removal of submicron-size particles required to achieve these efficiencies is impossible by these systems at high temperatures.
Although separators of various types have been suggested to obtain these higher fly ash removal efficiencies at low temperatures, including very elaborate versions of the precipitators mentioned above, they are large and expensive and their reliability is not as high as in previous designs because of their size and large number of components. Furthermore, gas turbine quality flue gas cleaned from the combustion of coal has not been accomplished yet without first cooling, then reheating the gases. Moreover, electric fields can be generated and maintained at high temperatures but only in laboratory conditions and without exposing the electrodes to the impact of high velocity solid particles, since, otherwise, they would erode in a very short time.